1. Field of the Invention
The present invention relates to structural assemblies and, more particularly, relates to a system and method for forming, bonding, or otherwise processing a preform in a vacuum vessel to form a structural assembly.
2. Description of Related Art
Superplastic forming (“SPF”) generally refers to a process for forming metals, including titanium, aluminum, and alloys of such metals, that exhibit superplastic behavior at certain temperatures, i.e., large elongations (up to about 2,000 percent). The SPF process can be used for forming a single SPF sheet or an SPF pack that includes multiple layered sheets. During the SPF process, the SPF sheet or pack is placed into a shaping die set and heated to a sufficiently high temperature within the superplasticity range of the material to soften the material. Pressurized gas is then injected against the material, and possibly into the pack, if applicable, thereby causing the sheet or pack to be urged against the dies. In some cases, portions of the sheets that form into contact are joined through brazing or diffusion bonding. The formed sheet or pack is then cooled and removed from the die set and final machining steps are performed, such as edge trimming. Advantageously, the SPF process can be used to form structures that can satisfy narrow shape and tolerance requirements without substantial additional machining. Superplastic forming is further described in U.S. Pat. Nos. 3,927,817; 4,361,262; 4,117,970; 5,214,948; 5,410,132; 5,700,995; 5,705,794; 5,914,064; 6,337,471, each of which is incorporated by reference.
In a conventional SPF process, the shaping die set includes first and second dies that cooperably define a die cavity, and which can be adjusted between open and closed positions. The dies are opened to receive the sheets or pack to be formed, and then closed for the forming operation. A hydraulically actuated press is used to maintain the dies in the closed position. That is, the dies are positioned in the press, and the press resists the force generated by the forming operation, which would otherwise open the dies during forming.
The press is typically a large device that requires a large workspace. In addition, the press is typically expensive, thereby adding to the cost for manufacturing parts by this operation. Further, the dies must be no larger than the maximum size that can be accommodated in the press. The geometry to be formed and the size of the dies are typically related to the number and thickness of the sheets that are to be formed. For example, a relatively larger die set is typically required to form multiple sheets simultaneously against the inner surfaces of the two opposed dies than is required for forming a single sheet against a single inner surface of the die set. Similarly, thicker sheets typically require dies of greater strength and, hence, greater size. Thus, the size of the press available for a production process may limit the type of parts that can be produced.
The press can also include an oven for heating the sheet or pack. In some cases, the oven is not sealed, and the sheet or pack is exposed to atmospheric elements during processing that can affect the resulting quality. Further, the entire oven is typically heated, even for processing small dies. The thermal mass of a large press can limit the speed at which the temperature can be adjusted, thereby preventing a reduction in processing time and possible improvement in quality that might result with faster temperature adjustments.
Alternatively, the dies can be connected to one another and prevented from opening, such as by pins inserted through bores that extend through interlocking connection portions of each of the dies. One such self-contained die is described in U.S. Pat. No. 5,823,034 to Nelepovitz. A preform assembly can be provided in the die, and the die can then be heated in a vacuum furnace, without requiring a press for maintaining the die in a closed configuration. However, the die must be specially formed with the connection portion, and the pins must be inserted and removed between forming operations.
Thus, there exists a need for an improved system and method for processing a preform to produce a structural assembly. The system and method should be capable of forming and/or bonding one or more members to form the assembly, and should be compatible with the production of large and/or complex structural members such as by superplastic forming, diffusion bonding, or brazing.